The present invention relates to heating of objects, and more particularly to methods and systems for heat-treating a workpiece.
Many applications require heating or annealing of an object or workpiece. For example, in the manufacture of semiconductor chips such as microprocessors and other computer chips for example, a semiconductor wafer such as a silicon wafer is subjected to an ion implantation process, which introduces impurity atoms or dopants into a surface region of a device side of the wafer. The ion implantation process damages the crystal lattice structure of the surface region of the wafer, and leaves the implanted dopant atoms in interstitial sites where they are electrically inactive. In order to move the dopant atoms into substitutional sites in the lattice to render them electrically active, and to repair the damage to the crystal lattice structure that occurs during ion implantation, it is necessary to anneal the surface region of the device side of the wafer by heating it to a high temperature.
However, the high temperatures required to anneal the device side also tend to produce undesirable effects using existing technologies. For example, diffusion of the dopant atoms deeper into the silicon wafer tends to occur at much higher rates at high temperatures, with most of the diffusion occurring within close proximity to the high annealing temperature required to activate the dopants. As performance demands of semiconductor wafers increase and device sizes decrease, it is necessary to produce increasingly shallow and abruptly defined junctions, and therefore, diffusion depths that would have been considered negligible in the past or that are tolerable today will no longer be tolerable in the next few years or thereafter. Current industry roadmaps, such as; the International Technology Roadmap for Semiconductors 1999 Edition (publicly available at http://public.itrs.net/) indicate that doping and annealing technologies will have to produce junction depths as shallow as 30 nm by 2005, and as shallow as 20 nm by 2008.
Most existing annealing technologies are incapable of achieving such shallow junction depths. For example, one existing rapid thermal annealing method involves illuminating the device side of the wafer with an array of tungsten filament lamps in a reflective chamber, to heat the wafer at a high rate. However, the wafer tends to remain hot for a considerable time after the power supply to the tungsten filaments is shut off, for a number of reasons. Typical tungsten lamps have a relatively long time constant, such as 0.3 seconds, for example, as a result of the high thermal masses of the filaments, which remain hot and continue to irradiate the wafer after the power supply to the filaments is discontinued. This slow time response of the filaments gives rise to the dominant thermal lag in such a system. Also, radiation return from the walls of the reflective process chamber provides another source of continued heating after the power is shut off. A temperature versus time profile of the wafer using this tungsten lamp annealing method tends to have a rounded top with relatively slow cooling after the power to the filaments is discontinued. Accordingly, if the wafer is heated with such a system to a sufficiently high temperature to repair the crystal lattice structure and activate the dopants, the wafer tends to remain too hot for too long a period of time, resulting in diffusion of the dopants to significantly greater depths in the wafer than the maximum tolerable diffusion depths that will be required to produce 30 nm junction depths.
Although the vast majority of dopant diffusion occurs in the highest temperature range of the annealing cycle, lowering the annealing temperature is not a satisfactory solution to the diffusion problem, as lower annealing temperatures result in significantly less activation of the dopants and therefore higher sheet resistance of the wafer, which would exceed current and/or future tolerable sheet resistance limits for advanced processing devices.
One annealing method that has achieved some success in producing shallow junctions involves the use of excimer lasers to heat and anneal the device side of the wafer. The short-wavelength monochromatic radiation produced by such lasers tends to be absorbed at very shallow depths in the device side of the wafer, and the short duration, high-power laser pulse (for example, a 10 nanosecond pulse delivering about 0.4 J/cm2 to the device side surface) typically used for this process tends to heat a small localized area of the surface of the device side to melting or near-melting temperatures very rapidly, in significantly less than the time required for thermal conduction in the wafer. Accordingly, the bulk regions of the substrate of the wafer tend to remain cold and therefore act as a heat sink for the heated surface region, causing the surface region to cool very quickly. A typical surface temperature versus time profile of the localized area of the device side surface using laser annealing tends to be triangular-shaped and steeply sloped for both the heating and cooling stages and therefore, the device side spends only a very short period of time at high temperatures. Thus, the wafer does not remain hot long enough for much dopant diffusion to occur. However, because the bulk regions of the wafer, as well as device side areas other than the localized area heated by the laser, remain cold when the localized surface area of the device side is heated to annealing temperature, extreme thermal gradients are produced in the wafer, resulting in large mechanical strains which cause the crystal planes within the wafer to slip, thereby damaging or breaking the crystal lattice. In this regard, a very small spatial movement may completely destroy the crystal lattice. Thermal gradients may also cause other damage, such as warpage or defect generation. Even in the absence of slippage, a non-uniform temperature distribution across the wafer may cause non-uniform performance-related characteristics, resulting in either inadequate performance of the particular wafer, or undesirable performance differences from wafer to wafer. In addition, the large amount of energy delivered by the laser or lasers to the device side of the wafer is non-uniformly absorbed by the pattern of devices thereon, resulting in deleterious heating effects in regions of the wafer where annealing is not desired, and may also produce further large temperature gradients causing additional damage to the silicon lattice.
Other ultra-fast heating methods similar to laser annealing have also been attempted. For example, flash lamps and microwave pulse generators have been used to rapidly heat the device side of the wafer to annealing temperature, resulting in a temperature-time profile similar to that achieved by laser annealing, with similar disadvantages.
At least one approach in the early 1990s involved a low-temperature annealing stage followed by a laser annealing stage. The low-temperature stage typically involved heating the wafer to a mid-range temperature in an electric furnace, such as 600xc2x0 C. for example, for a relatively long period of time, such as an hour or longer. A typical temperature-time profile of the device side surface using this method is flat for a very long time, followed by a rapid increase and rapid cooling of the surface resulting from the laser anneal. Although this method purports to reduce junction leakage currents as compared to laser annealing alone, the long duration of the low-temperature annealing stage causes the dopants to diffuse to greater depths within the device side of the wafer. Such diffusion, which may have been tolerable or perhaps even negligible by early 1990s standards, would not permit the formation of sufficiently shallow junctions to comply with current performance and industry roadmap requirements.
A more recent approach involves the use of a fast responding argon plasma arc lamp heat source to irradiate the substrate side of the wafer, to rapidly heat the entire wafer to annealing temperatures. The time response of the arc lamp is short (typically on the order of 0.1 milliseconds or less) compared to that of the wafer itself, and thus the dominant thermal lag is that of the wafer, in contrast with the tungsten lamp method above where the dominant thermal lag is that of the tungsten filaments. A typical temperature-time profile of the wafer using this method tends to have heating and cooling temperature rates that are intermediate between those of tungsten systems and laser annealing systems. Thus, the wafer spends less time at the high annealing temperature and therefore, less dopant diffusion occurs than with the tungsten lamp method. Accordingly, this method is capable of producing much shallower junction depths than tungsten lamp systems. As the entire wafer is heated rather than just the device side surface, the extreme transverse thermal gradients that result in laser annealing are avoided, thereby minimizing additional damage to the crystal lattice. In addition, as the substrate side is irradiated rather than the device side, non-uniform heating of the device side due to non-uniform absorption by the pattern of devices is also much lower than for laser annealing, resulting in lower lateral temperature gradients and reduced damage to devices. However, early indications suggest that embodiments of this method may result in somewhat deeper diffusion of the dopants than laser annealing.
Accordingly, there is a need for improved methods and systems for heat-treating a workpiece. In addition to annealing a semiconductor wafer for ion activation and lattice repair purposes, other applications would also benefit from an improved heat-treating method that addresses the above problems.
The present invention addresses the above needs by providing, in accordance with one aspect of the invention, a method and system for heat-treating a workpiece. The method involves increasing a temperature of the workpiece over a first time period to an intermediate temperature, and heating a surface of the workpiece to a desired temperature greater than the intermediate temperature. The heating is commenced within less time following the first time period than the first time period. The system includes a pre-heating device operable to increase the temperature of the workpiece and a heating device operable to heat a surface of the workpiece in the above manner. Similarly, an alternative system includes means for increasing the temperature and means for heating the surface in the above manner.
Increasing the workpiece temperature over the first time period, prior to commencing the heating of its surface, decreases the magnitude of the thermal gradients that will occur in the workpiece when the surface is heated to the desired temperature. Therefore, thermal stress in the workpiece is reduced, and where the workpiece has a crystal lattice structure, such as a semiconductor wafer for example, damage to the lattice is correspondingly reduced.
In addition, by commencing the heating within less time following the first time period than the duration of the first time period itself, the workpiece spends less time at high temperatures (in the general range of the intermediate temperature) than the time taken to increase the workpiece temperature to the intermediate temperature. Therefore, where the workpiece is a semiconductor wafer for example, the short duration of time spent at or approaching the intermediate temperature serves to minimize dopant diffusion that would otherwise occur if the workpiece spent longer times at such high temperatures.
Increasing the temperature of the workpiece preferably involves pre-heating the workpiece for a time period greater than a thermal conduction time of the workpiece. The pre-heating device may be operable to achieve this. This serves to allow much of the energy supplied to the workpiece during the preheating stage to conduct through the workpiece, thereby raising substantially the entire bulk of the workpiece to the intermediate temperature.
Similarly, heating preferably involves heating the surface for a time period less than a thermal conduction time of the workpiece. The heating device may be operable to achieve this. Thus, the surface may be heated quickly to the desired temperature while the bulk of the workpiece remains substantially at the cooler intermediate temperature. This allows the bulk of the workpiece to act as a heat sink for the heated surface, causing the surface to cool much more rapidly when the heating stage is completed. As dopant diffusion occurs most significantly at the highest temperature range, i.e. between the intermediate temperature and the desired temperature, this approach minimizes the time spent by the surface in this highest temperature range, thereby minimizing dopant diffusion.
Heating the surface of the workpiece preferably involves commencing the heating at an end of the first time period. The heating device may be operable to commence the heating at such a time.
Similarly, the method may further involve producing an indication of a temperature of the workpiece. Heating the surface may then involve commencing the heating in response to an indication that the temperature of the workpiece is at least the intermediate temperature. The system may include a temperature indicator operable to produce the indication of the temperature of the workpiece, and the heating device may be operable to commence the heating in response to the indication from the temperature indicator that the temperature of the workpiece is at least the intermediate temperature.
Thus, by commencing the heating of the surface as soon as the intermediate temperature is achieved, the workpiece spends even less time at the intermediate temperature, thereby further reducing the amount of dopant diffusion.
Increasing the temperature of the workpiece may involve irradiating the workpiece. In this regard, irradiating may involve exposing the workpiece to electromagnetic radiation produced by an arc lamp, or alternatively, to electromagnetic radiation produced by at least one filament lamp. Similarly, the pre-heating device may include means for irradiating the workpiece, or alternatively may include an irradiance source operable to irradiate the workpiece, and the irradiance source may include an arc lamp or at least one filament lamp, for example. Alternatively, the pre-heating device may include a hot body locatable to pre-heat the workpiece.
Increasing the temperature of the workpiece may involve pre-heating the workpiece at a rate of at least 250xc2x0 C. per second. Preferably, increasing involves pre-heating the workpiece at a rate of at least 400xc2x0 C. per second. The pre-heating device is preferably operable to pre-heat the workpiece at such rates. Such rapid pre-heating further reduces dopant diffusion, while allowing the temperature of the bulk of the workpiece to be increased in order to reduce thermal stresses and resulting lattice damage during subsequent heating of the surface. However, if desired, significantly slower or faster rates may be substituted.
Heating the surface of the workpiece preferably involves irradiating the surface. In this regard, irradiating may involve exposing the surface to electromagnetic radiation produced by a flash lamp, or alternatively, may involve moving a laser beam across the surface. Similarly, the heating device may include means for irradiating the surface, or may similarly include an irradiance source operable to irradiate the surface. The irradiance source may include a flash lamp or a laser, for example.
The method preferably further involves absorbing radiation reflected and thermally emitted by the workpiece. Absorbing may involve absorbing the radiation in a radiation absorbing environment, or alternatively, in at least one radiation absorbing surface. The method may further involve cooling the at least one radiation absorbing surface. Similarly, the system may further include a radiation absorbing environment, or alternatively, at least one radiation absorbing surface, operable to absorb radiation reflected and thermally emitted by the workpiece. The system may then include a cooling system operable to cool the at least one radiation absorbing surface.
If desired, increasing the temperature of the workpiece may further involve pre-heating the workpiece to the intermediate temperature, and heating the surface may involve heating the surface to a desired temperature greater than the intermediate temperature by an amount less than or equal to about one-fifth of a difference between the intermediate and initial temperatures. The heating device may be operable to perform such heating. In addition, or alternatively, increasing the temperature of the workpiece may involve irradiating a first side of the workpiece to pre-heat the workpiece to the intermediate temperature, and heating the surface may involve irradiating a second side of the workpiece to heat the second side to the desired temperature. In this regard, the pre-heating device and the heating device may include first and second irradiance sources operable to irradiate the first and second sides of the workpiece as indicated above. Advantages of these variations are discussed in connection with other aspects of the invention.
In accordance with another aspect of the invention, there is provided a method and system for heat-treating a workpiece. The method involves preheating the workpiece from an initial temperature to an intermediate temperature, and heating a surface of the workpiece to a desired temperature greater than the intermediate temperature by an amount less than or equal to about one-fifth of a difference between the intermediate and initial temperatures. The system may include a pre-heating device and a heating device operable to pre-heat the workpiece and to heat the surface of the workpiece respectively, in the above manner. An alternative system includes means for pre-heating the workpiece and means for heating the surface of the workpiece.
By pre-heating the workpiece in this manner, to an intermediate temperature that is relatively close to the desired temperature, the thermal gradients that are ultimately produced when the surface of the workpiece is heated to the desired temperature are reduced, resulting in lower thermal stresses in the workpiece. Where the workpiece has a crystal lattice structure such as that of a semiconductor wafer for example, this serves to reduce thermal stress damage to the lattice. At the same time, by pre-heating the workpiece to the intermediate temperature then heating the surface to the desired temperature, the surface may cool faster from the desired temperature than it would have if the entire workpiece had been heated to the desired temperature. Thus, the surface may spend less time at the high desired temperature, resulting in less dopant diffusion and therefore resulting in shallower junction formation.
Heating the surface to the desired temperature may involve heating the surface to a desired temperature greater than the intermediate temperature by an amount less than or equal to about one-tenth of the difference between the intermediate and initial temperatures. Similarly, the desired temperature may be greater than the intermediate temperature by an amount less than or equal to about one-twentieth of the difference between the intermediate and initial temperatures. The heating device may be operable to heat the surface in this manner. It has been found that these temperature relationships are particularly advantageous for some applications, such as ion activation in semiconductor wafers for example.
Similarly, the desired temperature may be at least about 1050xc2x0 C., which has been found to be a suitable ion activation annealing temperature for some applications. The heating device may be operable to heat the surface to such a temperature.
Pre-heating preferably involves pre-heating the workpiece for a time period greater than a thermal conduction time of the workpiece, and heating preferably involves heating the surface for a time period less than a thermal conduction time of the workpiece, as discussed above in connection with the previous aspect of the invention. The pre-heating and heating devices are preferably operable to perform such pre-heating and heating respectively.
Pre-heating preferably involves irradiating the workpiece, as discussed in connection with the previous aspect of the invention. Similarly, the preheating device may include means for irradiating the workpiece, or an irradiance source operable to irradiate the workpiece, or a hot body locatable to pre-heat the workpiece, as discussed above.
Pre-heating may involve pre-heating the workpiece at a rate of at least 250xc2x0 C. per second, and preferably at a rate of at least 400xc2x0 C. per second, which the pre-heating device may be operable to achieve.
Heating the surface of the workpiece preferably involves irradiating the surface, as discussed in connection with the previous aspect of the invention. Thus, the heating device may include means for irradiating the surface, or alternatively an irradiance source operable to irradiate the surface, as discussed above.
The method may further involve producing an indication of a temperature of the workpiece, and heating may then involve commencing the heating in response to an indication that the temperature of the workpiece is at least the intermediate temperature, as discussed above. The system may therefore include a temperature indicator and the heating device may be operable to commence the heating in response thereto, as discussed above.
The method may further involve absorbing radiation reflected and thermally emitted by the workpiece, as previously discussed, and thus, the system may include a radiation absorbing environment or radiation absorbing surface, and may further include a cooling system.
Pre-heating may involve irradiating a first side of the workpiece to pre-heat the workpiece to the intermediate temperature, and heating may involve irradiating a second side of the workpiece to heat the second side to the desired temperature. The pre-heating and heating devices may include first and second irradiance sources operable to irradiate the first and second sides of the workpiece, respectively.
In accordance with another aspect of the invention, there is provided a method and system for heat-treating a workpiece. The method involves irradiating a first side of the workpiece to pre-heat the workpiece to an intermediate temperature, and irradiating a second side of the workpiece to heat the second side to a desired temperature greater than the intermediate temperature. The system includes first and second irradiance sources operable to irradiate the first and second sides respectively in the above manner. An alternative system includes respective means for irradiating the first and second sides in the above manner.
It has been found that irradiating the first and second sides, to pre-heat the workpiece and to heat the second side respectively in the above manner, tends to reduce the magnitude of the thermal gradients that occur when the second side is heated to the desired temperature, thereby reducing damage to the workpiece resulting from thermal stresses. In addition, where the workpiece is a semiconductor wafer for example, the combination of preheating of the workpiece and heating of its second side serves to reduce dopant diffusion, thereby permitting the formation of shallower junctions than other technologies which attempt to provide relatively low thermal stress damage.
Irradiating the first and second sides preferably includes irradiating a substrate side and a device side respectively of a semiconductor wafer. The first and second irradiance sources may be locatable to irradiate the substrate and device sides respectively. Due to the greater uniformity of the emissivity across the substrate side of the wafer as compared to the device side, the irradiation of the substrate side to pre-heat the wafer results in significantly greater temperature uniformity in the wafer, and therefore significantly less thermal stress damage, than other methods that deliver the entire annealing energy to the device side of the wafer. In contrast, if the device side alone was irradiated to heat the device side from room temperature to 1050xc2x0 C. for example, then an emissivity difference of 10% between different devices on the device side may result in a lateral temperature difference of approximately 100xc2x0 C., which is well in excess of current tolerable temperature difference limits, and may therefore cause thermal stress damage to the devices and to the lattice.
Irradiating the first side preferably involves irradiating the first side for a time period greater than a thermal conduction time of the workpiece, and the first irradiance source may be operable to achieve this. This serves to allow much of the energy supplied to the first side of the workpiece during the pre-heating stage to conduct through the workpiece, thereby raising substantially the entire bulk of the workpiece to the intermediate temperature.
Conversely, irradiating the second side preferably involves irradiating the second side for a time period less than a thermal conduction time of the workpiece. The second irradiance source may be operable to irradiate the second side in this mariner. This allows the second side to be heated quickly to the desired temperature while the bulk of the workpiece remains substantially at the cooler intermediate temperature. The bulk of the workpiece may thus act as a heat sink for the heated second side, causing the second side to cool much more rapidly when the heating stage is completed. As dopant diffusion occurs most significantly at the highest temperature range, i.e. between the intermediate temperature and the desired temperature, this approach minimizes the time spent by the second side in this highest temperature range, thereby minimizing dopant diffusion.
Irradiating the first side preferably involves exposing the first side to electromagnetic radiation produced by an arc lamp, or alternatively, at least one filament lamp. Similarly, the first irradiance source may include means for irradiating the workpiece, or may include an arc lamp or at least one filament lamp.
Irradiating the first side may involve irradiating the first side with a radiation intensity sufficient to pre-heat the workpiece at a rate of at least 250xc2x0 C. per second, and preferably, at a rate of at least 400xc2x0 C. per second, as discussed in connection with a previous aspect of the invention. The first irradiance source may be operable to irradiate the first side at such rates.
Irradiating the second side preferably involves exposing the second side to electromagnetic radiation produced by a flash lamp, but may alternatively involve moving a laser beam across the surface. Similarly, the second irradiance source may include means for irradiating the workpiece, or may include a flash lamp or a laser.
The method may further involve producing an indication of a temperature of the workpiece, and irradiating of the second side may be commenced in response to an indication that the temperature of the workpiece is at least the intermediate temperature. Similarly, the system may include a temperature indicator and the second irradiance source may be operable to commence the irradiating of the second side in response such an indication from the temperature indicator.
The method may further involve absorbing radiation reflected and thermally emitted by the workpiece, as previously discussed, and similarly, the system may include a radiation absorbing environment, or may include at least one radiation absorbing surface, and may further include a cooling system.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.